I just wanted to correct one small thing. What I mean by relative error before is the residual norm, which is a relative number.
On Thursday, July 13, 2023 at 12:57:23 PM UTC+5:30 sabyasachi chatterjee wrote: > Hello, > > I am trying to solve a nonlinear contact problem using penalty method. The > tangent stiffness matrix is of the form K_e + c * K_p, where K_e and K_p > are the elastic and penalty contributions and c is the penalty parameter, > which is a large number (in the order of 10^2 to 10^3 in our case). The > part of the problem that carries out the linear solve for the incremental > displacement using Newton Raphson method is below: > > > **************************************************************************************** > PETScWrappers::MPI::Vector distributed_incremental_solution( > locally_owned_dofs, mpi_communicator); > distributed_incremental_solution = incremental_solution; > > SolverControl solver_control(20000, 1e-5); > PETScWrappers::SolverCG solver(solver_control, mpi_communicator); > PETScWrappers::PreconditionBlockJacobi preconditioner(system_matrix); > > solver.solve(system_matrix, > distributed_incremental_solution, > system_rhs,preconditioner); > > incremental_solution = distributed_incremental_solution; > > hanging_node_constraints.distribute(incremental_solution); > solution.add(1.0,incremental_solution); > > **************************************************************************************** > > I receive the following error upon running the code: > ------ > Iterative method reported convergence failure in step 33. The residual > in the last step was 0.000778898. > > This error message can indicate that you have simply not allowed a > sufficiently large number of iterations for your iterative solver to > converge. This often happens when you increase the size of your > problem. In such cases, the last residual will likely still be very > small, and you can make the error go away by increasing the allowed > number of iterations when setting up the SolverControl object that > determines the maximal number of iterations you allow. > > The other situation where this error may occur is when your matrix is > not invertible (e.g., your matrix has a null-space), or if you try to > apply the wrong solver to a matrix (e.g., using CG for a matrix that > is not symmetric or not positive definite). In these cases, the > residual in the last iteration is likely going to be large. > ------- > > For my problem, I don't require a relative error of the order of say > 10^(-12) as is often used in the tutorials and a relative error of 10^(-4) > probably should also work. I increased the relative error to 10^(-4) and > still got a similar error with the final residual a little more than > 10^(-4). I further increased the relative error to 10^(-3), but the > accuracy of the results got compromised. Let me know if there is any way to > go around this issue. > > Also please suggest if there is a way to NOT make the code stop if such a > situation arises and instead accept the result as it is and move ahead? > > Finally, do you recommend any other solver in this case . > > Thanks, > Sabyasachi > > > -- The deal.II project is located at http://www.dealii.org/ For mailing list/forum options, see https://groups.google.com/d/forum/dealii?hl=en --- You received this message because you are subscribed to the Google Groups "deal.II User Group" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/dealii/c3b3f77f-9083-4e61-9597-fe32ff3c10a0n%40googlegroups.com.
